The present invention relates generally to optical networking, and, more particularly, to installations in which a WDM ring carries short-haul communications.
There is currently a market need, experienced by, for example, telephone carriers and cable operators, to provide short-haul transmission of internet protocol (IP) packets among interconnected nodes that are typically spaced apart by about 20 km or less. Proposed systems use an IP packet over SONET interface, and use wavelength-division multiplexing (WDM) to carry the packets on an optical fiber transmission medium.
One particular class of architectures for the short-haul network is the class of ring architectures. Such architectures are especially useful for serving business parks, campuses, military bases, networks of geographically dispersed company buildings, and the like. Typically, a pair of counter-propagating fiber-optic rings connects a plurality of nodes, disposed along the ring, with a hub. The hub manages inbound and outbound transmissions between the ring and external communication networks. Each node typically serves one subscriber or aggregate of subscribers, which by way of illustration could be an office suite in an urban office building.
One drawback of optical ring networks is that the optical signal loss accrues over the network. Loss contributions include distributed loss due to attenuation in the optical fiber links that extend between adjacent nodes, and they also include discrete losses associated with the optical add-drop elements at the respective nodes.
Because these losses accrue over the network, such a network has a limited potential for growth. That is, even modest fractional increases in the total fiber length or the total number of nodes may carry an unacceptable loss penalty.
Conventionally, such a problem might be solved by providing amplification at each node. However, such a solution adds a substantial installation cost to the network. Since low cost is often one of the attractions of an optical ring network, this solution is likely to be unacceptable.
Therefore, there is a particular need for an optical ring network that can be expanded while maintaining relatively low overall cost.
We have developed a dual-ring, bi-directional optical fiber transmission system that interconnects a plurality of nodes with a hub, such that multiple WDM channels are established on each ring.
Although the invention is not so limited, this system is particularly useful when a relatively wide spacing of the channels, exemplary a spacing on the order of 10-30 nm, and more typically about 20 nm, enables the use of very low cost transceivers and avoids the need for temperature control. Such a WDM system is often referred to as a coarse WDM (C-WDM) system.
At each node, an optical add-drop module (OADM) operates to (a) extract, for the purposes of a receiver, or (b) insert, for the purposes of a transmitter, information in one or more of the channels.
We have found that accrued losses in this, and other, optical WDM networks can be offset in a cost-effective manner by distributed gain that is applied preferentially to those wavelength channels most in need of amplification. This helps to pave the way for network expansion while maintaining relatively low overall cost.
Accordingly, the invention involves an optical communication network that includes at least one unidirectional WDM ring that interconnects a hub and a plurality of nodes. A distinct wavelength channel is assigned to each node for communication along the ring from the hub to such node and from such node to the hub. Each adjacent pair of nodes is connected by a link of the WDM ring. The hub is likewise connected by a link to each of its adjoining nodes. At least one link, which is herein denominated a xe2x80x9cpumpedxe2x80x9d link, is optically coupled to a pump source. This coupling is direct, in the sense that it does not take place through the intermediacy of any intervening link. The aforesaid optical coupling of a pump source to a pumped link is arranged to optically amplify signals that pass through the pumped link.
Various arrangements of the pumped link or pumped links fall within the scope of the invention. In some embodiments of the invention, such arrangement has the property that: (i) signals that lie in a first wavelength channel and are associated with a first node will experience some optical gain when following a complete path, including at least one pumped link, between the hub and the associated node; and (ii) there will be signals lying in at least one further wavelength channel and associated with a further node that experience a different optical gain when following a complete path, including at least one pumped link, between the hub and the further node.
In alternate embodiments of the invention, there is at least one pumped link that imparts gain to signals in a first wavelength channel, and that imparts a different amount of gain to signals in a distinct, second wavelength channel.
In other embodiments of the invention, at least one pump source is a Raman pump source, arranged such that at least one wavelength channel is subject to Raman amplification in the corresponding pumped link.
In yet other, more specific embodiments of the invention, at least one pump source is a Raman pump source, arranged such that at least one wavelength channel will experience Raman gain in the corresponding pumped link, but also such that said gain is not experienced equally in all of the wavelength channels.